To compare the ABCG2 transporter activity between two TNBC cell lines resistant to ALA-PDT and PDT-sensitive breast cancer cell lines (T47D, SkBr3), cells were incubated with ABCG2 substrate pheophorbide a (Pha) with or without the presence of transporter inhibitor Ko143 and cell fluorescence of Pha was examined by flow cytometry. The gene discussed is ABCG2; the disease is breast cancer.